( * ... which is sorta Belgium-y German?* )
It's not Belgium-y German at all, just German. It's one of a few terms borrowed from French that exists across the entire language, akin to Croissant; while there are other ways to butter a Croissant so-to-speak ( Hörnchen, or in the Switzerland Gipfeli ), there is no reason to use these synonyms and in fact Croissant is more widespread and better understood. On point, Pommes is universal in German speech everywhere I have been or lived. Pommes Frites I only regularly see on menus, packaging, and advertising.
As for Kartoffelfrites, Frites is not from German, either, so if for the sake of argument the French pommes frites were to be directly transliterated into German, or so to speak Germanized, it would become Bratkartoffeln ... which is an entirely different dish.